Conventionally, in order for the door to be closed automatically, as shown in FIG. 2, a spring and a gear are installed inside the cylindrical-shaped body, which is filled with hydraulic oil. The restoring force of the spring transferred to the gear a 72, the gear b 73, and the link 74 allows the door to be closed slowly. In the conventional product, the restoring force of the spring is transferred through a gear and a link in order for the door to be closed so that a power loss is incurred and a very strong compression spring is required, and the structure is complicated and heavy because of the built-in gear, and the outer appearance may be spoiled due to the link attached outside. In addition, in order to adjust the opening and closing force of the door after installation, as in FIG. 2 and the link 74 is disassembled from the catch rod formed on the gear b 73, and the assembling angle of the link 74 to the catch rod 73a is changed to compress the spring 71, and the link is reinstalled for a force adjustment, so that it is very inconvenient to adjust the force, and a higher power is needed for the force adjustment. It must be installed by the specialist for the adjustment of the mounting position to the door and the doorframe and the length of the link. However, according to the present invention, the restoring force of the spring is directly transferred to the piston rod for closing the door without necessity of the gear. A small amount of air is contained in hydraulic oil to enable the piston rod to move forward and backward. A speed adjustment bolt is mounted to the edge of both sides of the cylinder for the door to be closed quickly or slowly. Also, a force adjustment nut, which is essentially required for installation of the present invention to the door and the doorframe, is assembled to one end of the piston rod to adjust opening and closing force when the door is opened and closed.